1. Field of the Invention
This invention relates to the field of theft prevention. More specifically, the invention comprises a system for detecting the presence of an identifying tag affixed to merchandise. The system includes novel features which greatly reduce the possibility of false alarms
2. Description of the Related Art
Theft detection systems have been in common use for many decades. One type of prior art system uses microwave transmissions to excite and detect a tag affixed to the merchandise to be protected. A system employing this technology is disclosed in U.S. Pat. No. 3,895,368 to Gordon (1975) and U.S. Pat. No. 4,063,229 to Welsh et. al. (1977).
The basic components of such systems are disclosed schematically in FIG. 1. Broadly, the system includes two transmitters. The first is a microwave transmitter operating in the region of 902 to 928 MHz. The second is a high voltage transmitter operating at much lower frequencies, such as 50 KHz. The two transmitters are equipped with appropriate antennas positioned to create a signal in the vicinity of a retail store's entrance/exit.
FIG. 5 illustrates a typical prior art installation. The objective is to provide surveillance of doorway 100 (typically the point of entry and departure for a retail sales establishment). An antenna for the microwave transmitter is located on each side of the doorway. Each antenna is housed within a microwave antenna housing 102. An antenna for the high voltage transmitter often spans most of the doorway's width. It can be located above the doorway, such as high voltage antenna 104 in FIG. 5. A control unit 106 is mounted somewhere nearby. It contains the systems disclosed in FIG. 1, along with power supplies and other conventional electronic components.
Returning to the prior art system shown in FIG. 1, the reader will also note the existence of the portion labeled receiver 46. This is the detecting side of the system. All these components interact with tags placed on the items within the store in order to determine when a tag is placed near the doorway (such as by a shoplifter exiting the store with stolen goods).
The tag is shown schematically in FIG. 2. Tag 66 contains a diode 68 attached to dipole element 70 and dipole element 72. The electronic components are housed within a durable tamper-proof housing (not shown). This housing is attached to the goods using tamper-proof pins, lanyards, or other known devices. When an item is lawfully purchased, a store clerk removes the tag using specialized equipment (in the case of reusable tags), or destroys the dipole circuit (in the case of disposable tags).
Those skilled in the art will realize that the system shown in FIG. 1 will create two fields in the area around the doorway: (1) an electromagnetic field produced by the high frequency microwave transmitter (902 to 928 MHz); and (2) an electrostatic field produced by the high voltage/low frequency transmitter (such as 50 KHz). Tag 66 is a passive electromagnetic wave receptor-reradiator with signal mixing capability. When a tag 66 is placed in the two fields, it will cause the radiation of two side bands. As an example, assume that the microwave transmitter is set for 915 MHz and the high voltage transmitter is set for 50 KHz. The dipole will emit side bands at 914.95 MHz (915 MHz−50 KHz) and 915.05 MHz (915 MHz+50 KHz), the sum and difference of the two signals. These side bands will typically only exist when a tag is near the doorway. Thus, these side bands can be used to indicate the presence of a tag.
The operation of the prior art device will now be described in greater detail. Transmitter 10 can be selected or set to transmit a microwave signal lying within the band between 902 and 928 MHz. For this example, assume the transmitter is set for 915 MHZ. The transmitter produces output signal 12. It may also produce low power reference signal 20 (having the same characteristics but much lower amplitude). Output signal 12 feeds into amplifier 14. The boosted signal is then fed into band-pass filter 16 (which is centered on 915 MHz). After passing the filter, the amplified signal is fed to antenna 18.
High voltage transmitter 48 starts with oscillator 50. In this example, an oscillator producing a 50 KHz signal is used. The oscillator is modulated by the output of FM modulator 112. In this example, a 1 KHz tone is used as the modulation signal.
The modulated signal is then amplified by amplifier 60. The signal then passed through step-up transformer 62 which substantially increases the voltage before feeding the signal to antenna 64. As explained previously, the antennas for both the microwave transmitter and the high voltage transmitter are placed to establish a signal in the region of the doorway.
Of course, the prior art system also incorporates a receiver with a detector. This portion is shown generally as receiver 46. Antenna 22 receives signals radiated by tag 66. These are then sent through band-pass filter 24 (typically centered on 915 MHZ). The filtered signal is then amplified by amplifier 26 before being fed into mixer 28.
Antenna 22 will pick up all signals in the vicinity of the doorway. Thus, it will pick up the 915 MHz microwave transmitter signal, the 50 KHz high voltage signal, and the two side bands (914.95 MHz and 915.05 MHZ) if a tag is present (Those skilled in the art will know that an antenna optimized for the 902 to 928 MHz band may receive little of the 50 KHz signal). Once the signal has traveled through band-pass filter 24, only the signals within the 902 to 928 MHz band will remain.
One of the functions of mixer 28 is to remove the 915 MHz signal, so that the presence of a side band can be more easily detected. A 915 MHz low power reference signal 20 can be fed into mixer 28 from transmitter 10 to establish a reference for the removal of the 915 MHz signal. Alternatively, a second and completely independent device can be used to feed a 915 MHz reference signal into mixer 28. However such a signal is established, it must be matched to the frequency of the transmitter's signal.
If a tag is present, mixer 28 will receive signals centered on 914.95 MHz, 915.00 MHz, and 915.05 MHz. The mixer then strips out the 915.00 MHz signal by conventional means. The result is that the two side band signals remain. After the removal of the 915.00 MHz signal, the two side bands will simply be two out-of-phase 50 KHz signals. These are then fed into FM detector 30, which is optimized for the detection of the 1 KHz modulation tone which is present on the 50 KHz signal. When a 1 KHz signal is detected by FM detector 30, it sends a 1 KHz signal to logic unit 32. Logic unit 32 is a conventional arrangement of logic circuits used to monitor the output of the FM detector. It maybe configured, as an example, to require that the FM detector produce a positive and steady signal for 0.5 seconds before it transmits a signal sounding alarm 40. The logic circuit thereby reduces false alarms caused by extraneous signals (which typically only exist for a short duration). The logic unit also allows the assembly to be reset to an inert state.
Those skilled in the art will know that devices such as shown in FIG. 1 often require a more complex antenna. FIG. 4 shows a more realistic antenna circuit for the microwave transmitter and receiver. A splitter 92 is added to the circuit of FIG. 1. It splits the microwave signal to be fed into two antennas 18.
The receiver is likewise modified to include two antennas 18. These two signals are fed into combiner 98, which then feeds the signal into band-pass filter 24. The balance of the receiver circuit is the same as in FIG. 1. The modification of FIG. 4 corresponds to the installation shown in FIG. 5, where two microwave antenna housings are shown.
Of course, the reader will appreciate that many other controls and features can be added to the system, including frequency tuning inputs, power settings, etc. However, the function of the prior art device is well illustrated by the components shown without adding undue complexity.
Over the years the prior art devices have been prone to high false alarm rates. Returning to FIG. 1, those skilled in the art will readily understand this problem. FM detector 30 is set to trigger an alarm when it receives a 50 KHz signal. The reader will recall that a 50 KHz signal is produced by high voltage transmitter 48. If the high voltage transmitter signal “leaks” into the receiver circuit, a false alarm will result.
No such leakage will occur in the schematic shown in FIG. 1. However, the reader will appreciate that the electrical isolation shown in the schematic is difficult to achieve in the real world.
In actual applications, “cross talk” is a fact of life. The 50 KHz high voltage signal (including the modulation signal) maybe inductively carried into antenna 22, the connective leads, the housing for the components, and many other features.
Users of such systems have had to resort to reducing the power of the 50 KHz signal to eliminate cross talk and resulting false alarms. In many installations, the power must be reduced to the point where the system can no longer reliably detect a tag.
The reader should appreciate that the cross talk problem is not unique to the selection of a 50 KHz high voltage signal in the example. If a 100 KHz signal is used, then FM detector 30 must be tuned to 100 KHz in order to detect the side band. Thus, the same cross talk problem persists, even though it occurs on the basis of a “false” 100 KHz signal rather than a “false” 50 KHz signal.
The prior art systems must generally be very carefully tuned in order to function at all. Returning to FIG. 5, the reader will recall that three different antenna housings are used. A skilled technician must typically place and tune these antennas in order to minimize the cross talk problem. This problem increases the cost of acquisition for the customer.